Optical networking (Dense Wavelength Division Multiplexing (DWDM) and the like) is driving for a continuous reduction in cost per bit along with increased capacity leading to a curve towards increasingly complex higher order modulation formats. These higher order modulation formats bring two related challenges, namely, 1) significantly higher Received Optical Signal-to-Noise Ratio (ROSNR) requirements and 2) modulation losses leading to transmit lower powers. Concurrently, flexibility in add/drop requirements has led to so-called colorless, directionless, and optionally contentionless add/drop multiplexer structures, such as in Reconfigurable Optical Add/Drop Multiplexer (ROADM) devices, nodes, architectures, and structures. A colorless add/drop device supports any wavelength being added on any port of an add/drop device, i.e., ports are not wavelength specific. A directionless add/drop device supports any port being directed to any degree. Finally, a contentionless add/drop device supports multiple instances of the same channel (wavelength) in the same device. A colorless, directionless add/drop device can be referred to as a CD device, and a colorless, directionless, and contentionless add/drop device can be referred to as a CDC device. Today's CD and CDC architectures rely on power combining to multiplex signals.
The above-referenced challenges are at odds with colorless multiplexer structures for CD or CDC devices. Specifically, colorless multiplexer structures that rely on the power combining of multiple signals. While this approach works well with robust modulation formats such as Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK), as modulation formats migrate forward to support rates of 200 Gb/s and above, this approach incurs significant penalties associated with accumulated Amplified Stimulated Emission (ASE) noise. Such penalties can result in worst case ROSNR requirements being crossed with higher order modulation formats before a single span is traversed due to the accumulated ASE noise. Also, the colorless multiplexer structures have an inherently high insertion loss. Without signal amplification prior to combining, power is too low when it reaches the following amplifier. This results in an unworkable noise penalty through the amplifier, again leading to an unworkable OSNR penalty.
Simply put, there is a need for CD and CDC architectures to support higher order modulation formats (e.g., supporting 200 Gb/s and above).